1. Technical Field
Embodiments of the invention relate to oxide substrates and manufacturing methods therefor.
2. Related Art
There has been concern in recent years that semiconductor devices may finally be facing the limits of the scaling law, which has been a guiding principle of performance advances in the field. In this context, materials are being developed that will make possible new operating principles to address the situation when the transistor limit is reached or approached. For example, in the field of spintronics, which exploits the spin degrees of freedom of electrons, there has been progress in the development of high-density non-volatile memories capable of high-speed operation at the same level as DRAM (dynamic random access memory).
There has also been progress in research into strongly-correlated electron system materials that cannot be described in terms of band theory, which is a cornerstone of semiconductor device design. Substances have been discovered that exhibit very large and rapid changes in physical properties caused by phase changes in the electron system. In strongly correlated electron system materials, a variety of electron phases with a variety of orders formed by spins, charges and orbitals are possible because the phase state of the electron system is affected not only by the spin degrees of the freedom but also by the degrees of freedom of the electron orbitals. Typical examples of strongly correlated electron system materials are the perovskite manganese oxides, in which a first order phase transition produces a charge-ordered phase by alignment of 3d electrons of manganese (Mn) and an orbital-ordered phase by alignment of the electron orbitals.
In a charge-ordered phase or orbital-ordered phase, electrical resistance increases because the carrier is localized, and the electron phase becomes an insulator phase. The magnetic behavior of this electron phase is that of an antiferromagnetic phase due to the superexchange interactions and double exchange interactions. The electron states of the charge-ordered phase and orbital-ordered phase should often be regarded as semiconductor states. This is because, although the carrier is localized in the charge-ordered phase and orbital-ordered phase, the electrical resistance is lower than that of a so-called band insulator. In accordance with convention, however, the electron phases of the charge-ordered phase and orbital-ordered phase are here called insulator phases. Conversely, when the behavior is metallic with low resistance, the electron phase is a ferromagnetic phase because the spins are aligned. The term “metallic phase” can be defined in various ways, but, as used in portions herein, a metallic phase is one in which “the temperature derivative of resistivity is positively signed.” Expressed in this way, the aforementioned insulator phase can be re-defined as one in which “the temperature derivative of resistivity is negative.”
A variety of switching phenomena have reportedly been observed in bulk single-crystal materials made of substances capable of assuming either the aforementioned charge-ordered phase or orbital-ordered phase, or a phase that combines both a charge-ordered phase and an orbital-ordered phase (charge- and orbital-ordered phase). See, e.g., Japanese Patent Application Publication Nos. H8-133894, H10-255481 and H10-261291, also referred to herein as Patent Documents 1-3, respectively.
These switching phenomena occur in response to applied stimuli, namely, temperature changes around the transition point, application of a magnetic or electric field, or light exposure. These switching phenomena are typically observed as very large changes in electrical resistance or transitions between antiferromagnetic and ferromagnetic phases. For example, resistance changes by orders of magnitude in response to application of a magnetic field are a well-known phenomenon called colossal magnetoresistance.
To achieve any kind of electronic device, magnetic device or optical device that uses these effects, the switching phenomena must be manifested when the perovskite manganese oxide has been formed as a thin film. As in the case of an ordinary semiconductor device, a single-crystal thin film with few defects is necessary in order to provide high-performance switching properties with little variation in properties. Research is therefore being done using laser ablation methods (or PLD methods), which allow the preparation of high-quality thin films of perovskite manganese oxides. Due to advances in film-forming technology, flat surfaces at the atomic layer level are now being formed in oxide single-crystal thin films. For example, it is now possible to control the atomic layers layer by layer when preparing perovskite manganese oxides if the intensity oscillation of the RHEED (reflection high-energy electron diffraction) is monitored.
To achieve any device with a high degree of utility using a perovskite manganese oxide, the switching phenomena must be manifested at room temperature or above, such as an absolute temperature of 300 K or more. However, the switching phenomena disclosed in the aforementioned documents have all been verified only under low-temperature conditions of about liquid nitrogen temperature (77 K) or less for example. In the perovskite manganese oxides disclosed in the aforementioned documents, the chemical composition can be represented by ABO3, with the atomic stacking planes stacked in a repeating pattern of AO layer, BO2 layer, AO layer . . . to form a stacked body. This kind of stacked crystal structure is represented hereunder as AO—BO2—AO. In the perovskite manganese oxides disclosed in the aforementioned documents, trivalent rare earth cations (hereunder represented as “Ln”) and a divalent alkaline-earth (“Ae”) randomly occupy the A sites in the crystal structure of the perovskite, and it is thought that the temperature at which the switching phenomena are manifested is lowered as a result of this randomness. It is known that the transition temperature for the charge-ordered phase can be elevated to about 500 K by ordering the A-site ions in an AeO—BO2—LnO—BO2—AeO—BO2—LnO—BO2. . . configuration. Regular arrangement of the ions occupying the A sites as in this example is called “A-site ordering” below. A feature of the group of substances exhibiting such high transition temperatures is that they contain Ba (barium) as an alkaline-earth Ae. For example, transition temperatures above room temperature have been reported with substances containing Ba as an alkaline-earth Ae and using Y (yttrium), Ho (holmium), Dy (dysprosium), Tb (terbium), Gd (gadolinium), Eu (europium) and Sm (samarium), which have small ionic radii, as the rare earth Ln.
For these switching phenomena to be applicable to an electronic device, magnetic device or optical device, they must be manifested when the oxide has been formed as a thin film. In a thin film formed on the surface of a substrate with a (100)-oriented surface ((100)-oriented substrate), the atomic stacking planes form an AO—BO2—AO stacked structure. With this AO—BO2—AO stacked structure, it is easy to order the A sites in a regular way. That is, a (100)-oriented substrate is convenient for ordering the A sites in the direction of film thickness. Conventionally, however, the problem has been that even if a single crystal of perovskite manganese oxide is formed as a thin film on a (100)-oriented substrate, the switching phenomena are not manifested in the resulting (100)-oriented perovskite manganese oxide single-crystal thin film. This is because the in-plane crystal lattice of the single-crystal thin film is fixed to the lattice of the substrate within the substrate plane, and the first order phase transition to a charge-ordered phase or orbital-oriented phase requires a kind of lattice deformation called Jahn-Teller deformation, which is suppressed by fourfold symmetry within the substrate plane.
On the other hand, Japanese Patent Application Publication No. 2005-213078(also referred to herein as “Patent Document 4”) discloses forming a perovskite oxide thin film formed using a (110) -oriented substrate. According to this disclosure, the formed thin film allows shear deformation of the crystal lattice during switching when the in-plane fourfold symmetry of the (110)-oriented substrate is broken. That is, in a thin film formed in accordance with Patent Document 4 the crystal lattice is oriented parallel to the substrate plane, while the charge-ordered plane or orbital-ordered plane is non-parallel to the substrate plane. As a result, first order phase transitions involving deformation of the crystal lattice are possible even with a single crystal thin film the crystal lattice of which is fixed to a lattice in an in-plane direction (referred to in portions herein as being “within a plane”) parallel to each atomic stacking plane of the substrate. Thus, according to Patent Document 4, a transition or in other words a switching phenomenon at high temperatures equivalent to those obtained with the bulk single crystal can be achieved by using a (110) -oriented substrate.
In a (110)-oriented thin film, the atomic stacking planes are stacked to make an (Ln,Ba)BO—O2—(Ln,Ba)BO stacked body. These atomic layer stacking planes are formed as follows. First, an (Ln,Ba)BO layer is formed, which is an atomic layer consisting of A sites containing a Ba atom or rare earth element Ln (represented as Ln,Ba), B sites and O atoms. An atomic layer containing two O atoms is formed next, followed by another (Ln,Ba)BO layer. In order to achieve A-site ordering in this (Ln,Ba)BO—O2—(Ln,Ba)BO stacked body, an order must be introduced into the A sites in the plane. However, for the sake of ordering the A sites within the plane some factor must provide a driving force for implementing regularity. In fact no such factor exists, and ordering the A sites in a (110)-oriented thin film is not an easy matter.